A vessel such as a bathtub or a sink is typically emptied through a drainage passage extending therethrough.
The drainage passage typically has a closure device associated with it. The closure device is typically moveable between a closed configuration which blocks fluid flow through the passage and an open configuration which allows fluid flow through the passage.
The passage is generally provided with a strainer which is basically an externally threaded tube insertable through the passage with a flange extending radially outwardly about an upper end thereof.
The strainer may include one or more members extending across the tube and defining apertures which are smaller in area than the cross sectional area of the tube. As the name suggests, the strainer will prevent objects above a certain size from passing through the tube. Commonly strainers have a “cross-hair” shaped member extending across the tube to define four apertures and a central opening which is threaded.
The flange or the strainer extends about the passage for sealing engagement with the vessel. The strainer is held in place by an internally threaded lower member which threadedly engages the tube and presses against the vessel to pull the flange tightly against the vessel.
The closure device typically interacts with the strainer. Early closure devices were simply plugs or stoppers removably insertable into the strainer to block the strainer. More modern closure devices interface with the upper flange rather than the strainer tube and utilize a gasket which is pressed against the flange to effect sealing in the closed configuration. Such devices typically include some mechanism for maintaining the gasket and the flange spaced apart in the open configuration.